1. Field of the Invention
The invention relates to a video amplifier for an image display device, which images are defmed by color signals, comprising, for each color signal, an amplifier circuit having a gain which is limited by a feedback assuming a higher value when the signal voltage exceeds a given threshold value, and a circuit called definition circuit, which is common for the assembly of colors, for establishing a threshold reference value.
The invention also relates to an image display device.
2. Description of the Related Art
A method, referred to as "soft clipping", in which the gain is progressively (soft) limited (clipping) in a video amplifier, is used, inter alia, for preventing contrast variations between images when the contents show sudden variations (for example, when sub-titles appear or disappear). The soft clipping attenuates the intensity of white and the modulation in the white areas, and precludes saturation of the video amplifier circuits.
A video processor comprising, for three colors, three amplifier circuits having a gain which is limited by a feedback assuming a higher value when the signal voltage exceeds a given threshold is known from the document U.S. Pat. No. 5,299,000. The processor described in this document comprises a threshold detection circuit which is common for the three colors and applies the same feedback simultaneously to the three amplifier circuits. Such a processor has the drawback that the same clipping threshold for the three amplifier circuits leads to a coloration of the white areas in certain circumstances.
To obviate this drawback, it has been proposed to use three circuits for defining distinct thresholds, one for each color. Nevertheless, this method is not always satisfactory because in the case of a relatively saturated color, i.e., when there is one strong color signal and the other two signals are weak color signals, the strong signal undergoes a feedback and is decreased, while the two other signals are not, which leads to a desaturation of the color. Moreover, as the cut-off voltages of the three cathodes of a display tube are fairly dispersed from tube to tube, and as it is common practice to control the three amplifier circuits one by one for adapting each of them to the cut-off voltage of the corresponding cathode, this will involve unforeseeable deviations from apparatus to apparatus between the feedback triggering point and the cut-off point.